As floating slag and soot are generated in a high-temperature furnace such as a gasification melting furnace, an ash melting furnace, or a refuse incinerator, clinker or slag (deposit) may be deposited and grow. For example, as illustrated in FIG. 11, due to the deposition and growth of the clinker or slag, a gas stream is obstructed and poor melting of fly ash occurs, or work of releasing the deposit by raising output of an auxiliary burner is required in the early stage of the deposition, or when the deposition and growth thereof are advanced to some extent, an operation is stopped and work of releasing the deposit using, for instance, a rammer is required. Further, since it is important to detect and deal with the deposit early on in order to increase work efficiency, in the related art, an observation window through which a deposition state of the deposit can be checked has been provided on a furnace. However, the entire inside of the furnace looks red due to high-temperature incinerated molten materials, and it is difficult to observe the deposit accurately through the observation window.
On the other hand, a procedure of directly inserting a water-cooled and air-cooled monitoring camera (device for monitoring the inside of a high-temperature furnace) into a furnace, capturing an image of the inside of the furnace, and thereby enable early detection of a deposit has been proposed and put to practical use (for example, see Patent Literatures 1 and 2).